Our basic aim is to improve our understanding of the complementary structural relations between steroid hormones and proteins that bind them with high specificity and high affinity. Soluble enzymes that act upon steroid hormones and can be isolated in pure form offer the best opportunity for study since their catalytic activity provides a sensitive probe for studying perturbation of the steroid binding site. Our program calls for the isolation and characterization of enzymes that act upon steroid hormones. Steroid derivatives that can serve as active site directed irreversible inhibitors will be used to map the steroid binding sites of these enzymes by labeling specific amino acid residues associated with those sites. Peptides containing the labeled amino acids will be isolated and their amino acid sequences determined. These studies in conjunction with the measurements of binding constants by equilibrium methods can be expected to yield important information concerning the amino acid environment associated with steroid binding sites. Such information has potential value in the design of enhancing and blocking agents that could be used for chemotherapy of hormone dependent tumors. This information could also be useful in the development of techniques of possible value in therapy.